1. Field of the Invention
The present invention relates to a photodetection apparatus such as an image sensor and so forth, and particularly to a photodetection apparatus with an improved dynamic range.
2. Description of the Related Art
In recent years, various imaging devices such as digital still cameras, digital video cameras, and so forth, employ a CCD (Charge Coupled Device), or a CMOS (Complementary Metal Oxide Semiconductor) image sensor.
Known examples of the advantages of employing a CMOS image sensor includes: the fact that CMOS image sensors can be manufactured with the same production line as with ordinary chips, and furthermore, such a CMOS image sensor can be formed together with a peripheral circuit in the form of a single chip; and the fact that the CMOS image sensor can operate at a lower voltage than that of CCDs, thereby exhibiting lower power consumption than that of CCDs.
Each pixel of the CMOS sensor comprises a single photodiode and switches using MOSFETs. That is to say, the CMOS sensor has a configuration in which photodiodes are arrayed in the form of a matrix, and switches are provided for each photodiode. With such an arrangement, charge is read out from the photodiodes in pixel increments by sequentially switching the switches. For example, a pixel circuit for such a CMOS image sensor is described in Non-Patent Document 1 (Ikebe et al. “Evaluation of a Functional Initializing for a CMOS-Image Sensor”, Technical Report of IEICE (The Institute of Electronics, Information and Communication Engineers), vol. 103, No. 298, pp. 19-24 (September 2003)).
FIG. 1 is a circuit diagram which shows a configuration of a pixel circuit 200 of a conventional CMOS image sensor described in FIG. 1 of the Non-Patent Document 1. The pixel circuit 200 includes a photodiode PD, a reset transistor M1, an amplifier transistor M2, and an output transistor M3. Each of the reset transistor M1, amplifier transistor M2, and output transistor M3, is an N-channel MOSFET. The reset transistor M1 and the photodiode PD are connected in series between the power supply voltage Vdd and the ground voltage GND. The source terminal of the reset transistor M1 is connected to the photodiode PD. On the other hand, the power supply voltage Vdd is applied to the drain terminal of the reset transistor M1. The gate terminal of the reset transistor M1 is an input terminal for receiving the reset signal RST.
The cathode terminal of the photodiode PD, connected to the reset transistor M1, is also connected to the gate terminal of the amplifier transistor M2. With regard to the amplifier transistor M2, the power supply voltage Vdd is applied to the drain terminal thereof, and the source terminal thereof is connected to the drain terminal of the output transistor M3. Thus, the amplifier transistor M2 functions as a source follower amplifier. The source terminal of the output transistor M3 is connected to a data line LD which is provided for each column of the CMOS image sensor.
With the image circuit 200 having such a configuration, upon the gate terminal of the reset transistor M1 receiving the high level reset signal RST, the reset transistor M1 is turned on. As a result, the power supply voltage Vdd is applied to the photodiode PD, thereby charging the cathode terminal of the photodiode PD with the power supply voltage. Then, the reset transistor M1 is turned off. In this state, upon the photodiode PD receiving light, photoelectric current flows, thereby discharging the charge accumulated in the cathode terminal of the photodiode PD. At this time, the voltage of the cathode terminal of the photodiode PD changes corresponding to the light intensity and the exposure time. The amplifier transistor M2 outputs the voltage of the cathode terminal of the photodiode PD.
After a predetermined exposure time, the selection signal SEL is switched to the high level. As a result, the output transistor M3 is turned on, and the voltage corresponding to the light amount received by the photodiode PD is output to the data line LD. Thus, an external circuit can read out the received light amount for each pixel.
Now, description will be made regarding the dynamic range of the pixel circuit of the conventional CMOS sensor shown in FIG. 1. As described above, the aforementioned pixel circuit detects the received light amount for each pixel as follows. First, the photodiode PD is charged with the power supply voltage Vdd. Then, the charge accumulated in the cathode terminal of the photodiode PD in the exposure time is discharged. Subsequently, the remaining charge amount is converted into a voltage, whereby the received light amount is measured. Let us consider a case in which the photodiode PD receives extremely high-intensity light. This leads to the remaining charge amount becoming zero in the exposure time. In this case, the pixel circuit 200 cannot completely detect the light amount input to the photodiode PD.
In order to solve the aforementioned problem, let us consider an arrangement in which the exposure time is reduced. Such an arrangement can detect higher-intensity light. However, such an arrangement is not sufficiently sensitive to detect a low-intensity light. As described above, the conventional pixel circuit 200 has a problem that the dynamic range is limited by the initial charge amount accumulated in the cathode terminal of the photodiode PD in the reset state. The initial charge amount is proportional to the power supply voltage Vdd and the capacity of the photodiode PD. Accordingly, with such a conventional arrangement, there is a need to increase the power supply voltage Vdd in order to increase the dynamic range. However, the increase in the power supply voltage Vdd negates the advantage of the CMOS image sensor, i.e., the ability to operate at a low voltage. Accordingly, such a countermeasure is not preferably employed.